Method for bonding stainless steel members and stainless steel

ABSTRACT

A method for bonding stainless steel members includes: contacting a first stainless steel member with a second stainless steel member that has a strain exceeding 50% reduction; and heating the first and second stainless steel members to a re-crystallization initiation temperature or higher, after the contacting.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a divisional of U.S. application Ser. No. 14/881,971filed Oct. 13, 2015, the entire contents of which is incorporated hereinby reference. U.S. application Ser. No. 14/881,971 is a continuationapplication of International Application PCT/JP2014/074972 filed on Sep.19, 2014 and designated the U.S., the entire contents of which areincorporated herein by reference.

FIELD

The present application relates to a method for bonding stainless steelmembers and a stainless steel.

BACKGROUND

A technology for bonding stainless steel members is being demanded. Forexample, Patent Document 1 discloses a technology in which surfaces ofmetal members are activated by removing oxides on bonding faces with achemical treatment and thereby a diffusion bonding is performed at a lowtemperature for suppressing grain coarsening.

PRIOR ART DOCUMENT Patent Document

Patent Document 1: Japanese Laid-open Patent Publication No. 2011-200930

SUMMARY

According to an aspect of the present invention, there is provided amethod for bonding stainless steel members including: contacting a firststainless steel member with a second stainless steel member that has astrain exceeding 50% reduction; and heating the first and secondstainless steel members to a re-crystallization initiation temperatureor higher, after the contacting.

According to another aspect of the present invention, there is provideda stainless steel that is obtained by bonding the first stainless steelmember and the second stainless steel member by the above-mentionedmethod for bonding stainless steel members.

According to another aspect of the present invention, there is provideda method for bonding stainless steel members including: contacting afirst austenitic stainless steel member with a second austeniticstainless steel member that contains at least 30 volume % of martensite;and heating the first and second austenitic stainless steel members toan As temperature or higher, after the contacting.

According to another aspect of the present invention, there is provideda stainless steel that is obtained by bonding the first austeniticstainless steel member and the second austenitic stainless steel memberby the above-mentioned method for bonding stainless steel members.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates a solid-phase diffusion bonding method in accordancewith a comparative embodiment:

FIG. 2 illustrates a bonding method in accordance with a firstembodiment;

FIG. 3 illustrates a flowchart of a bonding method in accordance with afirst embodiment;

FIG. 4 illustrates a flowchart of a bonding method in accordance with asecond embodiment;

FIG. 5 illustrates a result of a cross section around a bonding partobserved by EBSD:

FIG. 6 illustrates a relationship between temperatures during bondingand bonding strength:

FIG. 7 illustrates a relationship between temperatures during bondingand bonding strength;

FIG. 8 illustrates a relationship between temperatures during bondingand bonding strength.

DESCRIPTION OF EMBODIMENTS

Generally, a stainless steel is made by a smelting or a refining. Afterthat, a microstructure of the stainless steel is adjusted in order toprovide superior performance and is shipped. Generally, the stainlesssteel is processed into assemblies, and the assemblies are assembled.Thereby, various apparatuses are manufactured. There are various bondingtechnologies for assembling a stainless steel. However, a solid-phasediffusion bonding for contacting a bonding face with another bondingface in a solid phase and heating the bonding faces has to be used as abonding technology of small stainless steels that can be used and hasstrength at a relative low temperature and has a length of a fewmillimeters or less.

FIG. 1 illustrates a solid-phase diffusion bonding method in accordancewith a comparative embodiment. As illustrated in FIG. 1, a stainlesssteel member 10 and a stainless steel member 20 are compressed to eachother by a pressure device 30 at a high temperature. Thus, a stainlesssteel 40 that is a bonded assembly is obtained. In order to activatediffusion of constituent atoms sufficiently, a temperature of 1000degrees C. or higher is needed. However, with a high temperatureprocess, a problem may occur that coarsening of crystal grains occurs inthe stainless steel 40, and a problem may occur that strength of thestainless steel 40 is reduced by softening.

In order to solve the problems, it is necessary to achieve the diffusionbonding even if the temperature is low. For the purpose of solving theproblems, research and development are being performed in order toactivate bonding faces by cleaning the bonding faces The above-mentionedPatent Document 1 is an example. However, in a process using activationof a surface, when an activated surface is exposed to air, the activatedsurface is instantly contaminated by oxygen. And, the activated surfacemay be inactivated. Therefore, high quality bonding may not be achieved.It is thought that a sequence of processes from the surface treatment tothe bonding are performed in a vacuum chamber. However, in this case,cost may increase.

The present inventors focus attention on a phenomenon ofre-crystallization occurring in a final thermal process of processes foradjusting a structure of a stainless steel member. During thephenomenon, in a material, a newly generated crystal grain(re-crystallized grain) grows up introducing atoms into a surface from asurrounding material. The driving force is caused by a difference ofinternal energy between re-crystallized grains that have low internalenergy and are stable under an equilibrium state and re-crystallizedgrains that have high internal energy caused by strain or the like andare unstable. Based on the principle, when members are unstable byenhancing internal energy of the members, atoms near a metal surfacebecome more stable by crossing a bonding face and moving to a surface ofre-crystallized grains exposed from the other bonding face. Moreover,when the bonding faces disappear, energy becomes lower and bringsstability. As a result, the re-crystallized grains of the other bondingface cross the other bonding face and grow up. Thereby, an integratedstrong bonding is achieved. In this case, when the difference of theinternal energy of the both is sufficiently large, the growing of thecrystal grains progresses free from prevention by some contamination ofthe bonding faces.

First Embodiment

FIG. 2 illustrates a bonding method in accordance with a firstembodiment. FIG. 3 illustrates a flowchart of the bonding method. First,before heating for the bonding, at least one of the stainless steelmember 10 and the stainless steel member 20 is subjected to a reductionexceeding 50% to accumulate strain thereinside (Step S1). Next, bondingsurfaces of the stainless steel member 10 and the stainless steel member20 are smoothed (Step S2).

Next, the bonding surfaces of the stainless steel member 10 and thestainless steel member 20 are in touch with each other and are heated(Step S3). A heating temperature in this case is equal to are-crystallization initiation temperature or higher. When thetemperatures of the stainless steel member 10 and the stainless steelmember 20 are equal to the re-crystallization initiation temperature orhigher, re-crystallized grains are generated in the stainless steelmember 10 and the stainless steel member 20. Moreover, re-crystallizedgrains of the bonding face cross the bonding face and grow up, andstrong bonding is achieved. Thus, the stainless steel 40 is obtained.When the stainless steel member 10 and the stainless steel member 20 arepressed by the pressure device 30 and tightly adhere to each other inStep S3, stronger bonding is achieved.

In the embodiment, the strain is accumulated in at least one of thestainless steel member 10 and the stainless steel member 20 by addingreduction exceeding 50%. Thus, it is possible to achieve high qualitybonding at a relative low temperature that is equal to there-crystallization initiation temperature or higher. The crystal of thestainless steel 40 is miniaturized, because the bonding at the relativelow temperature is achieved. Therefore, it is possible to suppresssoftening of the stainless steel 40. Accordingly, it is possible tomanufacture a bonded assembly having high material strength and highspring characteristics. Even if the bonding surface is contaminated tosome extent because of adsorption of oxygen or the like, the bonding canbe achieved. It is therefore possible to perform the sequence ofprocesses in normal air except for the heating for the bonding.Moreover, it is possible to adjust the structure of the stainless steel40 to a necessary fine structure by a combination of a processing ofenhancing internal energy in advance and adding strain, the bondingtemperature and the process time. It is therefore possible to perform amanufacturing of materials and assembling of assemblies in parallel inthe sequence of processes. And, it is possible to contribute toefficiency of works and energy conservation, by omitting a thermalprocess to adjust a fine structure during a material manufacturing.

With the bonding method in accordance with the embodiment, the bondingof high quality can be achieved at a temperature that is equal to there-crystallization initiation temperature or higher. However, it ispreferable that the bonding is achieved at a temperature that is equalto the re-crystallization initiation temperature or higher and is equalto the re-crystallization initiation temperature plus 100 degrees C. orlower, from a viewpoint of suppression of coarsening of the crystalgrains. And, it is preferable that both of the stainless steel member 10and the stainless steel member 20 are subjected to the reductionexceeding 50% and the strain is accumulated in both of the stainlesssteel member 10 and the stainless steel member 20.

Second Embodiment

In a second embodiment, an effect caused by a phase transformation ofmetastable austenite-based stainless steel is used. FIG. 4 illustrates aflowchart of a bonding method in accordance with the second embodiment.In the embodiment, the metastable austenite-based stainless steel isused as the stainless steel members 10 and 20. In the embodiment, thedevice that is the same as the first embodiment can be used.

Before the heating for the bonding, strain is added to at least one ofthe stainless steel member 10 and the stainless steel member 20 at atemperature that is equal to an Md point or lower. Thereby, martensiteof 30 volume % or more is generated in the stainless steel member towhich the strain is added (Step S11). Martensite transformation isachieved by processing at the Md point or lower.

Next, the bonding surfaces of the stainless steel members 10 and 20 aresmoothed (Step S12). Next, the bonding surfaces of the stainless steelmembers 10 and 20 are in touch with each other and are heated (StepS13). The temperature of this case is equal to an As point of thestainless steel members 10 and 20 or higher. The martensite istransformed into austenite when the martensite is heated to the As pointor higher. When the temperatures of the stainless steel members 10 and20 are equal to the As point or higher, re-crystallized grains aregenerated inside the stainless steel members 10 and 20. Moreover, at abonding face, the re-crystallized grains cross the bonding face and growup. And, a strong bonding is achieved. Thus, the stainless steel 40 isobtained. When the stainless steel members 10 and 20 are compressed toeach other by the pressure device 30 and adhere to each other, strongerbonding is achieved.

In the embodiment, at least one of the metastable austenite-basedstainless steel members 10 and 20 contains at least 30 volume % ofmartensite. Thereby, high quality bonding can be achieved at a relativelow temperature that is equal to the As point or higher. By achievingthe bonding at a low temperature, the structure of the stainless steel40 is miniaturized and softening of the stainless steel 40 issuppressed. It is therefore possible to manufacture a bonded assemblythat has high material strength and high spring characteristic. Even ifbonding faces are contaminated in some degree because of adsorption ofoxygen, bonding can be achieved. It is therefore possible to perform asequence of processes in normal air, except for the heating for thebonding. Moreover, it is possible to adjust the structure into anecessary fine structure by combining of a process of generating themartensite, the bonding temperature and the process time. It istherefore possible to perform a manufacturing of materials andassembling of assemblies in parallel in the sequence of processes. Andit is possible to contribute to efficiency of works and energy saving byomitting a thermal process for adjusting of a microstructure in amanufacturing of materials.

In the bonding method in accordance with the embodiment, high qualitybonding can be achieved at a temperature that is equal to the As pointor higher. However, it is preferable to achieve the bonding at atemperature that is equal to the As point or higher and is equal to are-crystallization initiation temperature plus 100 degrees C. or lower,in terms of suppressing coarsening crystal grains. It is preferable thatat least one of the stainless steel members 10 and 20 contains at least50 volume % of martensite. It is more preferable that at least one ofthe stainless steel members 10 and 20 contains at least 80 volume % ofmartensite. Both of the stainless steel members 10 and 20 may contain atleast 30 volume % of martensite. In this case, it is preferable thatboth of the stainless steel members 10 and 20 contain at least 50 volume% of martensite. It is more preferable that both of the stainless steelmembers 10 and 20 may contain at least 80 volume % of martensite.

In the embodiment, the martensite is generated by the process at the Mdpoint or lower. However, the martensite may be generated by rapidlycooling stable austenite to the Ms point or lower. When stable austeniteis rapidly cooled to the Ms point or lower, the martensite is generated.

EXAMPLES First Example

Austenite-based stainless steel SUS316L was subjected to 99% reductionat a normal temperature (that is equal to the Md point or higher) byforging and rolling. The resulting board having a thickness of 1 mm wascut into two small pieces having a width of 12 mm and a length of 20 mm.First faces of the small pieces were processed into mirrored faces byemery paper and buffing in the atmosphere. The mirrored faces are madeto face with each other and arranged crosswise in a vacuum chamber.Thus, the faces of 12 mm×12 mm adhered to each other. After vacuuming,the pieces were heated by high frequency heating to 730 degrees C. thatis equal to a re-crystallization initiation temperature or higher undera condition that a load of 1 kN was added to the pieces in order toachieve adherence of the pieces. After the temperature of 730 degrees C.under the condition was kept for 30 minutes, the pieces were unloaded,cooled and brought out. Thereby, the pieces were strongly bonded. Forconfirmation, one of the pieces bonded crosswise was fixed by a vise,and the other was hammered to a dragging away direction. This results inbending of the piece fixed by the vise without dragging away.

Second Example

In a second example, the same bonding as the first example except foradding 80% rolling was performed. In the second example, a strongbonding was achieved.

First Comparative Example

In a first comparative example, the same bonding as the first exampleexcept for adding 50% rolling was performed. The resulting bondedassembly was dragged away by hammering.

Second Comparative Example

In a second comparative example, the same bonding as the first examplewas performed except for using SUS316L material in which strain wascompletely removed and was subjected to a solution treatment. Theresulting bonded assembly was easily dragged away by a hand.

(First Analysis) In the first and second examples, it is thought thatre-crystallized grains crossed a bonding face and grew up and highquality bonding was achieved because strain was accumulated in thepieces by adding reduction exceeding 50% and, after that, the bondingwas performed at a temperature that is equal to the re-crystallizationinitiation temperature or higher. On the other hand, in the secondcomparative example, it is thought that strong bonding was not achievedbecause the strain was completely removed. In the first comparativeexample, it is thought that sufficient bonding strength was not achievedbecause of lack of strain although given bonding strength was achievedby accumulation of internal strain.

Third Example

In a third example, metastable austenite-based stainless steel SUS304was subjected to totally 99% strain at 300 degrees C. (that is equal tothe Md point or higher) by multi-direction forging and rollingcorresponding to 90% and rolling corresponding to 90%. The resultingboard having a thickness of 1 mm was cut into two small pieces having awidth of 12 mm and a length of 20 mm. First faces of the small pieceswere processed into mirrored faces by emery paper and buffing in theatmosphere. The mirrored faces are made to face with each other andarranged crosswise in a vacuum chamber. Thus, the faces of 12 mm×12 mmadhered to each other. After vacuuming, the pieces were heated by highfrequency heating to 730 degrees C. that is equal to are-crystallization initiation temperature or higher under a conditionthat a load of 1 kN was added to the pieces in order to achieveadherence of the pieces. After the temperature of 730 degrees C. underthe condition was kept for 30 minutes, the pieces were unloaded, cooledand brought out. Thereby, the SUS304 pieces were strongly bondedalthough, normally, a temperature of approximately 1000 degrees C. wasneeded for diffusion bonding of SUS304 pieces.

Cross sections near the bonding part were observed by EBSD (ElectronBackscatter Diffraction). FIG. 5 illustrates the observation result. InFIG. 5, the bonding part is between a left arrow and a right arrow.Although the cross sections were smoothed before bonding, unevennesscaused by grain growth from up and down was observed.

Fourth Example

In a fourth example, austenite-based stainless steel SUS316L wassubjected to 99% reduction at a normal temperature (that is equal to theMd point or higher) by forging and rolling. The resulting board having athickness of 1 mm was cut into two small pieces having a width of 10 mmand a length of 50 mm. First faces of the small pieces were processedinto mirrored faces by emery paper and buffing in the atmosphere. Themirrored faces are made to face with each other and arranged crosswisein a vacuum chamber. Thus, the faces of 10 mm×10 mm adhered to eachother. After vacuuming, the pieces were heated by high frequency heatingto various temperatures under a condition that a load of 1 kN was addedto the pieces through a punched hole having a diameter of 5 mm in orderto achieve adherence of the pieces. After the temperatures under thecondition was kept for 30 minutes, the pieces were unloaded, cooled andbrought out. For confirmation, a cross tension test of 0.01 mm/s wasperformed. And bonding strength was evaluated.

Third Comparative Example

In a third comparative example, the same bonding as the fourth examplewas performed except for using SUS316L material in which strain wascompletely removed and was subjected to a solution treatment.

(Second Analysis) FIG. 6 illustrates a relationship between thetemperatures during the bonding of the fourth example and the thirdcomparative example and the bonding strength. In FIG. 6, a horizontalaxis indicates the temperatures during the bonding. A vertical axisindicates the bonding strength. W99 indicates the results of the fourthexample. SOL indicates the results of the third comparative example. Asillustrated in FIG. 6, in the both cases, the bonding strength tends toincrease as the temperature during the bonding increases. However, theresults of the fourth example are shifted to a lower temperature side,compared to the third comparative example. That is, it is apparent thatin the fourth example, the temperature during the bonding can be reducedin order to achieve identical bonding strength, compared to the thirdcomparative example.

Fifth Example

In a fifth example, metastable austenite-based stainless steel SUS304was subjected to 90% reduction at 300 degrees C. (that is equal to theMd point or higher) by multi-direction forging and rolling and wassubjected to 90% rolling at a normal temperature (that is equal to theMd point or higher). Almost entirely the resulting board was transformedinto martensite and had a thickness of 1 mm. The resulting board was cutinto two small pieces having a width of 12 mm and a length of 20 mm.First faces of the small pieces were processed into mirrored faces byemery paper and buffing in the atmosphere. The mirrored faces are madeto face with each other and arranged crosswise in a vacuum chamber.Thus, the faces of 12 mm×12 mm adhered to each other. After vacuuming,the pieces were heated by high frequency heating to 700 degrees C. thatis equal to the As point or higher under a condition that a load of 1 kNwas added to the pieces in order to achieve adherence of the pieces.After the temperature of 700 degrees C. under the condition was kept for30 minutes, the pieces were unloaded, cooled and brought out. Thereby,the SUS304 pieces were strongly bonded although, normally, a temperatureof approximately 1000 degrees C. was needed for diffusion bonding ofSUS304 pieces. For confirmation, one of the pieces bonded crosswise wasfixed by a vise, and the other was hammered to a dragging awaydirection. This results in bending of the piece fixed by the visewithout dragging away.

Fourth Comparative Example

In a fourth comparative example, the bonding test was performed underthe same condition as the fifth example with use of a SUS304 sample thatwas subjected to totally 99% strain by multi-direction forging androlling corresponding to 90% and rolling corresponding to 90% at 300degrees C. The rolling was not performed at a temperature that is equalto the Md point or lower. Therefore, martensite was not generated. Whenone of the members bonded crosswise was fixed by a vise and the other ofthe members was hammered in a dragging away direction, the other wasdragged away.

Fifth Comparative Example

In a fifth comparative example, the test was performed under the samecondition as the fourth comparative example except for using the samesize SUS304 to which strain was not added after the a thermal processfor removing the strain.

(Third Analysis) In the fifth example, it is thought thatre-crystallized grains crossed a bonding face and grew up and highquality bonding was achieved because the martensite of 30 volume % ormore was generated and the bonding was performed at the temperature thatis equal to the As point or higher. On the other hand, in the fourth andfifth comparative examples, it is thought that strong bonding was notachieved because the martensite was not generated. In the fourthcomparative example, it is thought that strong bonding was not achieved,because the temperature was not increased to the re-crystallizationtemperature or higher although the strains was added.

Sixth Example

In a sixth example, metastable austenite-based stainless steel SUS304was subjected to 90% reduction at 300 degrees C. (that is equal to theMd point or higher) by multi-direction forging and rolling and wassubjected to rolling corresponding to 90% at a normal temperature (thatis equal to the Md point or lower). Almost entirely the resulting boardwas transformed into martensite and had a thickness of 1 mm. Theresulting board was cut into two small pieces having a width of 10 mmand a length of 50 mm. First faces of the small pieces were processedinto mirrored faces by emery paper and buffing in the atmosphere. Themirrored faces are made to face with each other and arranged crosswisein a vacuum chamber. Thus, the faces of 10 mm×10 mm adhered to eachother. After vacuuming, the pieces were heated by high frequency heatingto various temperatures under a condition that a load of 1 kN was addedto the pieces through a punched hole having a diameter of 5 mm in orderto achieve adherence of the pieces. After the temperatures under thecondition was kept for 30 minutes, the pieces were unloaded, cooled andbrought out. For confirmation, a cross tension test of 0.01 mm/s wasperformed. And bonding strength was evaluated.

Seventh Example

In a seventh example, the same bonding as the sixth example wasperformed except for using SUS304 material in which strain wascompletely removed and was subjected to a solution treatment, as one ofthe two small pieces.

Sixth Comparative Example

In a sixth comparative example, the same bonding as the seventh examplewas performed except for using SUS304 material in which strain wascompletely removed and was subjected to a solution treatment, as the twosmall pieces.

(Fourth Analysis) FIG. 7 illustrates a relationship between thetemperatures during the bonding of the sixth and seventh examples andthe sixth comparative example and the bonding strength. In FIG. 7, ahorizontal axis indicates the temperatures during the bonding. Avertical axis indicates the bonding strength. WC/WC indicates theresults of the sixth example. WC/SOL indicates the results of theseventh example. SOL/SOL indicates the results of the sixth comparativeexample. As illustrated in FIG. 7, in any cases, the bonding strengthtends to increase as the temperature during the bonding increases.However, the results of the sixth and seventh examples are shifted to alower temperature side, compared to the sixth comparative example. Thatis, it is apparent that in the sixth and seventh examples, thetemperature during the bonding can be reduced in order to achieveidentical bonding strength, compared to the sixth comparative example.

Eighth Example

In an eighth example, austenite-based stainless steel SUS304 wassubjected to 99% reduction at 300 degrees C. (that is equal to the Mdpoint or higher) by forging and rolling. The resulting board having athickness of 1 mm was cut into two small pieces having a width of 10 mmand a length of 50 mm. First faces of the small pieces were processedinto mirrored faces by emery paper and buffing in the atmosphere. Themirrored faces are made to face with each other and arranged crosswisein a vacuum chamber. Thus, the faces of 10 mm×10 mm adhered to eachother. After vacuuming, the pieces were heated by high frequency heatingto various temperatures under a condition that a load of 1 kN was addedto the pieces through a punched hole having a diameter of 5 mm in orderto achieve adherence of the pieces. After the temperatures under thecondition were kept for 30 minutes, the pieces were unloaded, cooled andbrought out. For confirmation, a cross tension test of 0.01 mm/s wasperformed. And bonding strength was evaluated.

Ninth Example

In a ninth example, the same bonding as the eighth example was performedexcept for rolling of 80%.

(Fifth Analysis) FIG. 8 illustrates a relationship between thetemperatures during the bonding of the eighth example, the ninth exampleand the sixth comparative example and the bonding strength. In FIG. 8, ahorizontal axis indicates the temperatures during the bonding. Avertical axis indicates the bonding strength. W99 indicates the resultsof the eighth example. W80 indicates the results of the ninth example.SOL/SOL indicates the results of the sixth comparative example. Asillustrated in FIG. 6, in any cases, the bonding strength tends toincrease as the temperature during the bonding increases. However, theresults of the eighth and ninth examples are shifted to a lowertemperature side, compared to the sixth comparative example. That is, itis apparent that in the eighth and ninth examples, the temperatureduring the bonding can be reduced in order to achieve identical bondingstrength, compared to the sixth comparative example.

The present invention is not limited to the specifically describedembodiments, but other embodiments and variations may be made withoutdeparting from the scope of the claimed invention.

What is claimed is:
 1. A method for bonding stainless steel memberscomprising: contacting a first austenitic stainless steel member with asecond austenitic stainless steel member that contains at least 30volume % of martensite; and heating the first and second austeniticstainless steel members to an As temperature or higher, after thecontacting.
 2. The method for bonding stainless steel members as claimedin claim 1, wherein the first austenitic stainless steel member containsat least 30 volume % of martensite.
 3. The method for bonding stainlesssteel members as claimed in claim 1, wherein in the heating, the firstaustenitic stainless steel member and the second austenitic stainlesssteel member are heated to a temperature that is equal to the Astemperature or higher and is equal to a re-crystallization initiationtemperature plus 100 degrees C. or lower.